Wireless receiver

ABSTRACT

A wireless receiver includes an RSSI generation circuit that obtains RSSI output corresponding to a carrier strength level of a received RF signal; a lookup table from which a threshold value corresponding to temperature information from a temperature sensor is read based on the temperature information; a comparison circuit that generates comparison output when the RSSI output is below the threshold value read from the lookup table, in which the threshold value is one input, and the RSSI output from the RSSI generation circuit is the other input; and a muting circuit that closes a signal line of an audio signal demodulated from the RF signal, and cuts off output of the audio signal, based on the comparison output from the comparison circuit. The above configuration enables the wireless receiver to eliminate fluctuation of a reception reaching distance relative to temperature change, and ensure stable mute operation.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a wireless receiver that receives a radio wavefrom, for example, a wireless microphone and demodulates an audiosignal, and in particular relates to a wireless receiver capable ofpreventing fluctuation of a reception reaching distance (whichdetermines an area where a combination system of a wireless microphoneand a receiver can be used) due to temperature dependence of thereceiver.

Description of the Related Art

For example, in a wireless receiver that receives a modulated RF signal(hereinafter referred to as an RF signal) transmitted from a wirelessmicrophone and demodulates an audio signal, audibly unpleasant noise isgenerated when transmission of the RF signal (in particularFrequency-Modulated RF signal, hereinafter referred to as FM-RF signal)from the wireless microphone is stopped, or in a case where a receptionstate of the RF signal is deteriorated.

In order to remove such noise, this kind of wireless receiver includes amuting circuit (also referred to as a squelch circuit).

As one of the muting circuits, an RF muting circuit has been proposedthat opens and closes a signal line of a demodulated signal according toa level of the received RF signal. The RF muting circuit operates toclose the signal line of the demodulated audio signal and to reduceoutput of the audio signal when the level of the received RF signal isbelow a predetermined threshold value.

That is, to operate the RF muting circuit, a unit that obtainsinformation (received signal strength indication: RSSI) according to thelevel of the received RF signal, and a comparison unit that compares avalue of the RSSI with a predetermined threshold value are included.

However, the value of the RSSI largely depends on temperature of afront-end including an RF amplification circuit and a local oscillationcircuit, and the like. Consequently, with the above-described mutingcircuit using the RSSI having the temperature dependence, an operatingpoint of the muting circuit naturally fluctuates due to the temperature.For this reason, a reception reaching distance (which determines an areawhere the system can be used) of the combination system of the wirelessmicrophone and the wireless receiver fluctuates (varies) due tooperating temperature of the receiver.

In order to reduce such temperature dependence, a wireless receiver hasbeen proposed in which a countermeasure is applied such as correction ofthe value of the RSSI corresponding to the operating temperature, andthe receiver has been disclosed in, for example, JP 2006-270902 A(Patent Document 1)and JP 2014-017782 A (Patent Document 2).

The receiver disclosed in JP 2006-270902 A uses a constant currentcircuit in which an output current value is changed according to thetemperature and corrects the temperature dependence of the RSSI, therebysuppressing variation of the operating point of the muting circuitaccording to the temperature.

The receiver disclosed in JP 2014-017782 A arranges, in the front-end, acorrection circuit including a thermistor which changes its gaindepending on the temperature, thereby correcting the temperaturedependence of the RSSI similarly, and suppressing variation of theoperating point of the muting circuit.

That is, both receivers correct the output of the RSSI having thetemperature dependence with the constant current circuit or the circuitincluding the thermistor both having the temperature dependencesimilarly, and apply the value (voltage value) of the corrected RSSI tothe comparison circuit, and compare the value with a fixed mutereference voltage (threshold voltage).

FIG. 8 is a graph describing mute operation in the conventionalreceiver, and the left vertical axis indicates the mute referencevoltage, the right vertical axis indicates the RSSI voltage, and thehorizontal axis indicates the operating temperature. A characteristicdenoted by a indicates an output characteristic of the RSSI beforecorrection of the temperature dependence, and a characteristic denotedby b indicates an example of an output characteristic of the RSSI aftercorrection of the temperature dependence. A dashed line denoted by cindicates the fixed mute reference voltage (threshold voltage).

By the way, it is very difficult to perform correction so that thetemperature characteristic is completely eliminated for thecharacteristic a of the RSSI before the correction, by using theconstant current circuit or the like as disclosed in JP 2006-270902 Aand JP 2014-017782 A, within a practical temperature range in which thiskind of wireless receiver is used. That is, there are many examples inwhich the value of the RSSI becomes a gradient curve as illustrated bythe corrected output characteristic b.

In this example, compared to an execution temperature t1 of the muteoperation of the output a before the correction, an executiontemperature of the mute operation of the output b after the correctionincreases to t2, and the reception reaching distance extendssubstantially. However, in the output b after the correction, the muteoperation is executed at the temperature t2, but the mute operation isnot executed at the temperature t1, and the reception reaching distance(which determines an area where the system can be used) fluctuates dueto the temperature.

SUMMARY OF THE INVENTION

The invention has been made based on the above-described technical pointof view, and an object thereof is to provide a wireless receiver capableof ensuring stable mute operation by suppressing fluctuation of areception reaching distance (within which a system can be used) relativeto temperature change, while using the RSSI output having a temperaturecharacteristic for mute operation.

The wireless receiver according to the present invention that has beenmade to solve the above-described problem, includes: an RSSI generationcircuit that generates RSSI output corresponding to a carrier strengthlevel of a received RF signal; a lookup table from which a thresholdvalue corresponding to temperature information from a temperature sensoris read out based on the temperature information; a comparison circuitthat generates comparison output when the RSSI output is below thethreshold value read from the lookup table or a value based on thethreshold value, for which comparison circuit the threshold value or thevalue based on the threshold value is one input, and the RSSI outputfrom the RSSI generation circuit is the other input; and a mutingcircuit that closes a signal line of an audio signal demodulated fromthe RF signal, and cuts off output of the audio signal, based on thecomparison output from the comparison circuit.

In this case, in a preferable form, a configuration is adopted in whicha practical temperature range in which the wireless receiver is used isdivided into a plurality of stages, and the threshold value is stored inthe lookup table in correspondence to each temperature range dividedinto the plurality of stages.

In another preferable form, a configuration is adopted in which the oneinput of the comparison circuit is a value linearly interpolated byusing the threshold value read from the lookup table.

The wireless receiver according to the invention can suitably adopt acircuit configuration in which a thermistor is used for the temperaturesensor.

An inclination of the above-described RSSI output relative totemperature and an inclination of the threshold value corresponding tothe temperature information of the lookup table are set to be equal toeach other. Further, an amount of temperature drift of the RSSI outputfrom the normal temperature is set to be the same as an amount oftemperature drift of the threshold value from the normal temperaturecorresponding to temperature information in the lookup table.

With the wireless receiver of the above-described configuration, theRSSI output corresponding to the carrier strength level of the receivedRF signal and the threshold value read from the lookup table aresupplied to the comparison circuit. The level of the RSSI output iscompared to the threshold value, and operation of the muting circuit isselected based on the comparison result.

In this case, in the wireless receiver according to the presentinvention, based on the temperature information from the temperaturesensor, an appropriate threshold value is read from the lookup table andis compared to the RSSI output including the temperature characteristic.

Accordingly, the above configuration allows to cancel the temperaturecharacteristic of the RSSI output by using the threshold value accordingto the temperature stored in the lookup table, and as a result, toachieve the mute operation that does not depend on the temperature.

Thus, the wireless receiver can be provided which enables to eliminatethe fluctuation of the reception reaching distance (which determines anarea where the system can be used) relative to the temperature changeand to ensure the stable mute operation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram illustrating an example of a wireless receiveraccording to the invention;

FIG. 2 is a block diagram illustrating a first example of a mute controlcircuit in the wireless receiver illustrated in FIG. 1;

FIG. 3 is a block diagram similarly illustrating a second example of themute control circuit;

FIG. 4 is a graph illustrating a first operation example of the mutecontrol circuit illustrated in FIG. 2;

FIG. 5 is a graph illustrating a second operation example of the mutecontrol circuit illustrated in FIG. 2;

FIG. 6 is a graph illustrating an operation example of the mute controlcircuit illustrated in FIG. 3;

FIG. 7 is a graph comparing a reception reaching distance of thewireless receiver; and

FIG. 8 is a graph illustrating an operation example of a mute controlcircuit in a conventional wireless receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A wireless receiver according to the present invention will be describedbased on embodiments illustrated in the drawings.

An example described below is a wireless receiver that receives an FM-RFsignal from a wireless microphone as a transmitter and demodulates anaudio signal, and FIG. 1 illustrates an entire configuration of thewireless receiver in a block diagram.

In FIG. 1, an RF signal from a receiving antenna (not shown) is fed toan RF amplification circuit 1. The RF signal from the RF amplificationcircuit 1 is fed to a first frequency conversion circuit 2.

The first frequency conversion circuit 2 receives a signal from a firstlocal signal oscillator (not shown) by a PLL synthesizer, for example,and generates a first intermediate frequency signal (first IF signal)from the RF signal and a first local signal. That is, by selection ofthe first local signal by the PLL synthesizer, the first intermediatefrequency signal corresponding to a particular receiving frequency isgenerated, and is supplied to an intermediate frequency amplificationcircuit 3.

The first IF signal amplified by the intermediate frequencyamplification circuit 3 is supplied to a second frequency conversioncircuit 4. The second frequency conversion circuit 4 receives a fixedlocal signal from a second local signal oscillator (not shown), andgenerates a second intermediate frequency signal (second IF signal) fromthe first intermediate frequency signal (first IF signal) and a secondlocal signal. That is, the wireless receiver adopts a doublesuperheterodyne system.

The second intermediate frequency signal (second IF signal) isdemodulated to the audio signal by a demodulation circuit 5 including alimiter amplifier, and is supplied to a muting circuit 7 through anaudio processing circuit 6. The muting circuit 7 has a function ofopening and closing a signal line of a demodulated signal (audiosignal), after receiving a control signal from a mute control circuit 9which will be described later. The demodulated signal passing throughthe muting circuit 7 is outputted to an audio output terminal 8.

Meanwhile, an FM-IF signal is supplied from the demodulation circuit 5to the mute control circuit 9, and the control signal from the mutecontrol circuit 9 is supplied to the above-described muting circuit 7.

FIG. 2 illustrates a first configuration example of the mute controlcircuit 9 shown in FIG. 1. The mute control circuit 9 includes a leveldetection circuit 11 that receives the FM-IF signal from thedemodulation circuit 5 illustrated in FIG. 1, and performs leveldetection of the signal. Level detection output from the level detectioncircuit 11, that is, the RSSI output corresponding to the carrierstrength level of the RF signal is supplied to an inverting inputterminal of a comparison circuit 12 including a comparator and the like.

A threshold output (threshold voltage) read from a lookup table 13 isfed to a non-inverting input terminal of the comparison circuit 12.

The lookup table 13 has a function of reading out a threshold voltagecorresponding to the temperature information, based on the temperatureinformation from temperature sensor. That is, a thermistor TH is usedfor the temperature sensor according to the embodiment as illustrated inFIG. 2, and, to the thermistor TH, a resistive element R1 is connectedin series and a DC voltage E1 is applied. A DC voltage generated acrossthe thermistor TH is extracted as the temperature information, and thetemperature information is subjected to digital conversion by an A/Dconverter 14, and is sent to the lookup table 13.

Data of the threshold voltages aa V to zz V corresponding totemperatures aa° C. to zz° C. are stored in the lookup table 13. Thethreshold voltage data corresponding to the temperature informationgenerated by the thermistor TH read from the lookup table 13 becomes athreshold voltage generated by a D/A converter 16. The comparisoncircuit 12 compares the threshold voltage with the RSSI output. As aresult, the comparator generates a comparison output when the RSSIoutput becomes lower than the threshold voltage, and supplies thecomparison output to the muting circuit 7 as a mute control signal.Thus, the muting circuit 7 operates to close the signal line of theaudio signal and to shut off the output of the audio signal.

FIG. 4 illustrates an operation example of the mute control circuit 9illustrated in FIG. 2. That is, in FIG. 4, the left vertical axisindicates the mute reference voltage, the right vertical axis indicatesthe RSSI voltage, and the horizontal axis indicates the operatingtemperature. The solid line a indicates the RSSI voltage havingtemperature dependence, and the dashed line b indicates the thresholdvoltage (mute reference voltage) read from the lookup table 13. At thistime, the solid line a and the dashed line b have the same inclination.As illustrated in FIG. 4, when let a drift voltage in RSSI voltage(solid line a) at each temperature from the RSSI voltage at the normaltemperature denote ΔV, if the mute reference voltage (dashed line b) isset to change the same drift voltage of ΔV, then at any temperature themute reference voltage (dashed line b) will have a constant voltagedifference ΔV with respect to the RSSI voltage (solid line a).

Therefore, making the above setting enables to eliminate variation ofmute operation relative to temperature change, and allows to provide thewireless receiver without variation due to the temperature change of thereception reaching distance (which determines the area where the systemcan be used).

For example, in a case where transmission of the RF signal from thewireless microphone stops, or in a case where a reception frequencyradio wave is suppressed by disturbance due to interference ofintermodulation, for example, the muting circuit 7 operates and allowsnormal mute operation to cut off audibly unpleasant noise.

FIG. 5 illustrates another operation example of the mute control circuit9 illustrated in FIG. 2, and the relationship between the vertical axisand the horizontal axis in FIG. 5 is the same as the relationship of theexample which is already described illustrated in FIG. 4.

In the example illustrated in FIG. 5, the practical temperature range(horizontal axis in FIG. 5) in which the wireless receiver is used isdivided into, for example, five stages, and threshold voltages (mutereference voltages) b1 to b5 are set in correspondence to the respectivetemperature ranges divided into five stages.

Therefore, with the above setting, a small scale lookup table sufficesfor the lookup table 13, and a comparable effect can be obtained inpractical use to the example illustrated in FIG. 4 with the lookup table13 having a large number of stored data.

FIG. 6 illustrates another operation example of the mute control circuit9, and the relationship between the vertical axis and the horizontalaxis in FIG. 6 is the same as the relationship of the exampleillustrated in FIG. 4 already described.

In the example illustrated in FIG. 6, the practical temperature range(horizontal axis in FIG. 6) in which the wireless receiver is used isdivided into, for example, five stages, and threshold voltages (mutereference voltages) b1 to b5 are set in correspondence to the respectivetemperature ranges divided into five stages.

However, each of the threshold values b1 to b5 illustrated in FIG. 6 hasa characteristic that is linearly inclined, and uses a linearlyinterpolated value by using upper limit and lower limit threshold valuesread from the lookup table 13. The solid line a and each of thethreshold values b1 to b5 have the same inclination.

The second configuration example of the mute control circuit 9 thatachieves the operation is illustrated in FIG. 3. In a blockconfiguration illustrated in FIG. 3, interpolation processing 15 isadded to the configuration illustrated in FIG. 2, and, since otherelements are the same as those in FIG. 2, overlapping description isomitted.

That is, in the lookup table 13 in FIG. 3, the threshold value data ofthe upper limit and lower limit respectively corresponding to thethreshold values b1 to b5 illustrated in FIG. 6 are stored for each ofthe temperature ranges of five stages. For example, regarding thethreshold value b1, an upper limit threshold voltage a V and a lowerlimit threshold voltage d V are stored for a temperatures range from ato d° C.

In correspondence to the temperature range obtained by the temperaturesensor including the thermistor TH, the above-described threshold valuedata of the upper limit and lower limit are read simultaneously. Thethreshold value data of the upper limit and lower limit are linearlyinterpolated in the interpolation processing 15 illustrated in FIG. 3,and a threshold voltage (mute reference voltage) corresponding to eachtemperature is calculated, and a threshold value corresponding to thethreshold voltage is supplied to the comparison circuit 12.

Therefore, also in a case where the mute control circuit 9 describedbased on FIGS. 3 and 6 is used, a small scale lookup table suffices forthe lookup table 13, and a comparable effect can be obtained inpractical use to the example illustrated in FIG. 4 with the lookup table13 having a large number of stored data.

Data of the threshold voltages aa V to zz V corresponding totemperatures aa° C. to zz° C. are stored in the lookup table 13.Threshold voltage values corresponding to a temperature range more thanaa° C. to less than zz° C. (excluding both end temperatures aa° C. andzz° C.) may be calculated by linear interpolation of interpolationprocessing 15.

FIG. 7 represents a relationship between the operating temperature andthe reception reaching distance for the conventional wireless receiverand the wireless receiver according to the present invention. Thevertical axis in FIG. 7 indicates the reception reaching distance(within which the system can be used), and the horizontal axis indicatesthe operating temperature.

A characteristic a indicates a reception reaching distance of theconventional wireless receiver using output of the RSSI beforecorrection of the temperature dependence corresponding to thecharacteristic a in FIG. 8. A characteristic b indicates a receptionreaching distance of the conventional wireless receiver using output ofthe RSSI after correction of the temperature dependence corresponding tothe characteristic b in FIG. 8.

A characteristic c indicates a reception reaching distance of thewireless receiver according to the present invention including thethreshold voltage (mute reference voltage) corresponding to thecharacteristic b in FIG. 4, for example.

With the wireless receiver illustrated as a conventional example, thereception reaching distance is decreased in a high-temperature region asillustrated in the characteristics a and b. This is because the RSSIvoltage is decreased in the high-temperature region and is below a fixedmute reference voltage, and the muting circuit operates early.

On the other hand, with the wireless receiver according to the presentinvention, little change is observed in the reception reaching distancewithin the practical temperature range as illustrated in thecharacteristic c.

This is a result of reading an appropriate threshold voltage (mutereference voltage) from the lookup table, based on the temperatureinformation, and utilizing the threshold voltage to cancel thetemperature characteristic of the RSSI output substantially. Thus, thewireless receiver can be provided capable of eliminating fluctuation ofthe reception reaching distance relative to temperature change, andensuring stable mute operation.

In the above-described embodiments, description has been made based onthe wireless receiver that receives and demodulates the RF signal fromthe wireless microphone; however, the wireless receiver according to thepresent invention can obtain a similar effect when adopted in otherwireless receivers that receive and demodulate the RF signal, besidesthe wireless receiver that receives the RF signal from theabove-described particular transmission source.

The level detection circuit 11 and the comparison circuit 12 describedin the embodiments can be replaced by a configuration using digitalprocessing.

What is claimed is:
 1. A wireless receiver comprising: an RSSIgeneration circuit that obtains RSSI output corresponding to a carrierstrength level of a received RF signal; a lookup table from which athreshold value corresponding to temperature information from atemperature sensor is read based on the temperature information; acomparison circuit that generates comparison output when the RSSI outputis below the threshold value read from the lookup table or a value basedon the threshold value, wherein the threshold value or the value basedon the threshold value is one input, and the RSSI output from the RSSIgeneration circuit is the other input; and a muting circuit that closesa signal line of an audio signal demodulated from the RF signal, andcuts off output of the audio signal, based on the comparison output fromthe comparison circuit.
 2. The wireless receiver according to claim 1,wherein a practical temperature range in which the wireless receiver isused is divided into a plurality of stages, and the threshold value isstored in the lookup table in correspondence to each temperature rangedivided into the plurality of stages.
 3. The wireless receiver accordingto claim 2, wherein the one input of the comparison circuit is a valuelinearly interpolated by using the threshold value read from the lookuptable.
 4. The wireless receiver according to claim 1, wherein athermistor is used for the temperature sensor.
 5. The wireless receiveraccording to claim 1, wherein an inclination of the RSSI output relativeto temperature and an inclination of the threshold value correspondingto the temperature information of the lookup table are equal to eachother.
 6. The wireless receiver according to claim 1, wherein an amountof temperature drift of the RSSI output from the normal temperature isthe same as an amount of temperature drift of the threshold value fromthe normal temperature corresponding to temperature information in thelookup table.